1. Field of the Invention
The present invention relates generally to polymeric materials that are re-mendable and self-healing. More particularly, the present invention is directed to thermally re-mendable polymeric materials that are made from multi-valent furan monomers and multi-valent maleimide monomers via the Diels-Alder (DA) reaction.
2. Description of Related Art
The publications and other reference materials referred to herein to describe the background of the invention and to provide additional detail regarding its practice are hereby incorporated by reference. For convenience, the reference materials are numerically referenced and grouped in the appended bibliography.
In past decades, highly cross-linked polymers have been studied widely as matrices for composites, foamed structures, structural adhesives, insulators for electronic packaging, etc. (1, 2). The densely cross-linked structures are the basis of superior mechanical properties such as high modulus, high fracture strength, and solvent resistance. However, these materials are irreversibly damaged by high stresses (3, 4) due to the formation and propagation of cracks. The latter lead to dangerous loss in the load-carrying capacity of polymeric structural engineering materials (5, 6).
The exploration of re-mending and self-healing of polymeric materials has become increasingly more exciting in the recent past, culminating in a genuinely self-healing material (4). The hot plate welding and crack healing of thermoplastics, where intermolecular noncovalent interactions (chain entanglements) at the interface are responsible for mending, have been well established (7-9). Small molecules-induced crack healing has also been studied for thermoplastics (10, 11), and a composite of a linear polymer with a thermoset that has crack-healing was reported (12). The concept of self-repair was introduced to heal cracks by embedding hollow fibers that can release repair chemicals when a crack propagates (7, 13, 14). The most recent report of crack self-healing (autonomic healing) of an epoxy resin consists of a clever use of catalytic network formation of an encapsulated add-monomer, which is held within a capsule embedded in the epoxy matrix (4). But questions remain concerning the long-term stability of the catalyst and the ability of the material to self-heal multiple times.
There presently is a need for truly “re-mendable” polymers that are highly cross-linked, transparent and that exhibit multiple cycles of autonomic crack mending with simple, uncatalyzed thermal treatment wherein covalent bonds are formed at the interface of the mended parts. This is to be contrasted with hot plate welding and crack healing of thermoplastics where only intermolecular, noncovalent interactions (chain entanglements, hydrogen bonding, etc.) are responsible for the mend and no new covalent bonds are formed between the mended parts.